1. Field of the Invention
The present invention relates to a method and device for adjusting a multi-beam source unit used in an image forming apparatus such as a digital copying machine or a laser printer, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit.
2. Description of the Prior Art
Heretofore, as an image forming apparatus such as a digital copying machine or a laser printer there has been known an image forming apparatus with a laser scanning optic system mounted thereon. With the recent tendency to a higher write accuracy and a higher write speed, a laser scanning optical system using a multi-beam laser diode is becoming most popular.
FIG. 1 illustrates such a laser sunning optical system schematically. In the same figure, the reference numeral 1 denotes a multi-beam source unit, numeral 2 denotes a polygon mirror, numeral 3 denotes a fxcex8 lens, and numeral 4 denotes a photosensitive member (also called an image recording medium). The multi-beam source unit 1 substantially comprises a multi-beam laser diode 5 and a collimator lens 6. The multi-beam laser diode 5 has a plurality of light emitting points to emit multi-laser beams P. The multi-laser beams P are collimated by means of the collimator lens 6, then are reflected by the polygon mirror 2 and are conducted onto a surface (also called an image recording surface) 4a of the photosensitive member 4.
The polygon mirror 2 and the fxcex8 lens 3 constitute a part of a scanning optical system. On the surface 4a of the photosensitive member 4, as show in FIG. 2, the multi-laser beams P are scanned in a horizontal direction Q1 while leaving predetermined pitches X1 in a vertical scanning direction Q2 which is perpendicular to the horizontal scanning direction Q1. In this type of a laser scanning optical system, the surface 4a of the photosensitive member 4 is scanned over a large number of lines at a time to write data onto the surface 4a. 
As the write accuracy and speed have recently become higher, the laser scanning optical system is required to be improved in its accuracy for the diameter of a beam spot 11 of each multi-laser beam P, collimatability of the beams, a pitch X1 between adjacent beam spots 11 in the vertical scanning direction Q2, and a write start position in the horizontal scanning direction Q1. The accuracy required is becoming more and more strict as a higher image quality is required
As shown in FIG. 3, the multi-beam laser diode 5 has a light emitting section 7 in the interior thereof. In the light emitting section 7 are provided a plurality of light emitting points, which are, for example, four light emitting points 7a to 7d. The light emitting points 7a to 7d are arranged on a designwise predetermined virtual straight line Q3 spacedly from one another. The virtual straight line Q3 is obtained by joining acute points 9a and 10a of a pair of acute-angled cutout portions 9 and 10 which are formed in a metallic stem 8 of the multi-be laser diode 5.
In the conventional multi-beam laser diode 5, the light emitting points 7a to 7d are spaced widely from one another, so that when multi-laser beams are projected onto the surface 4a of the photosensitive member 4, the pitches X1 of their beam spots 11 in the vertical scanning direction Q2 become large and the image quality becomes coarse. For avoiding such an inconvenience, as shown in FIG. 4, the multi-beam liner diode 5 is turned around the optic axis of the scanning optical system (not shown) so that an arranged direction (straight line) Q3xe2x80x2 of the beam spots 11 becomes oblique with respect to the horizontal scanning direction Q1 on the surface 4a of the photosensitive member 4, thereby adjusting the pitches X1 in the vertical scanning direction Q2 to enhance the write density (recording density) in the vertical scanning direction Q2 and improve the image quality.
However, if the multi-beam laser diode 5 is turned so that the arranged direction (straight line) Q3xe2x80x2 of the beam spots 11 is deviated obliquely with respect to the vertical scanning direction Q2, to improve the write density, then in the case where the light emitting points 7a-7d are driven simultaneously to effect write, this results in that write start positions in the horizontal scanning direction Q1 of the beam spots 11 on the surface 4a of the photosensitive member 4 become displaced and hence the image quality is deteriorated.
In the laser scanning optical system of this type, for making the write start positions of the beam spots on the surface 4a of the photosensitive member 4 uniform, for example sensors 12 for detecting scan positions of the laser beams are arranged correspondingly to the laser beams and the emission of beam from each of the light emitting points 7a to 7d is controlled in accordance with a light receiving timing of each sensor 12.
More particularly, the emission of light from the light emitting points 7a-7d is controlled upon the lapse of time t0xe2x80x2 after the detection of the head beam spot 11 at time t=t0, thereby making write start positions in the horizontal scanning direction Q1 on the surface 4a of the photosensitive member 4 uniform.
Moreover, instead of using sensors 12 correspondingly to the light emitting points 7a-7d a sensor 12 is provided correspondingly to the head beam spot 11 in the horizontal scanning direction Q1, as shown in FIG. 5(a), then time lags t1, t2, and t3 of beam spots 11 are determined in advance, and as shown in FIG. 5(b), after the head beam spot 11 has been detected by the sensor 12, the emission of beams from the remaining light emitting points 7b-7d is delayed correspondingly to the time lags by means of a delay control circuit (not shown), thereby making the beam spots 11 uniform in the vertical scanning direction at the write start positions on the surface 4a of the photosensitive member 4, as shown in FIG. 5(a).
In this laser scanning optical system, however, the control it used for aligning write start positions is complicated, with consequent increase of cost.
At present, a multi-beam laser diode 5 with light emitting points 7a-7d spaced more narrowly than before is being developed. In a multi-beam source unit having such a multi-beam laser diode 5 it is presumed that positional variations of the light emitting points 7a-7d will be small. Further, the multi-beam source unit is probably designed so as to be set to a scanning optical system on the assumption that light emitting points 7a-7d are arranged in the direction of a predetermined standard design line when they are present on the virtual straight line Q3 defined by a pair of cutout portions 9 and 10, and is attached as it is to a body portion of an image forming apparatus.
Even with such a design however, due to errors in the manufacturing process of the multi-beam laser diode 5 being considered, it is rarely the case that the light emitting points 7a-7d are positioned on the virtual straight line Q3 without error. Even if there exists an arrangement direction (straight line) Q4 joining light emitting points 7a-7d, as shown in FIG. 6, the arrangement direction Q4 and the virtual straight line Q3 are slightly inclined with respect to each other and thus aligning the arrangement direction of the light emitting points 7a-7d with the direction of the standard design line without the need of any adjustment is difficult. The reference mark xcex8 represents the angle of that inclination.
When the multi-beam laser diode 5 is to be mounted to a body portion of an image forming apparatus body with a scanning optical system mounted thereon, since there exists a mounting error, it is desirable to turn the multi-beam laser diode 5 around the optical axis so that the arrangement direction Q4 of the light emitting points is at a predetermined angle relative to the vertical scanning direction Q2.
The present invention has been accomplished in view of the above circumstances and it is an object of the invention to provide a multi-beam source unit adjusting method and device wherein an arranged direction of light emitting points of a multi-beam laser diode relative to a horizontal scanning direction of a scanning optical system can be aligned with a predetermined standard design line direction, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit, particularly, a multi-beam source unit adjusting method and device wherein an arranged direction of light emitting points of a multi-beam laser diode can be adjusted to a vertical scanning direction of a scanning optical system easily without impairing a required design accuracy, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit.
For achieving the above object, according to the present invention, in the first aspect thereof, there is provided a method for adjusting a multi-beam source unit, the multi-beam source unit including a multi-beam laser diode capable of emitting multi-laser beams from a plurality of light emitting points and a collimator lens for collimating the multi-laser beams, the multi-laser diode having a stem formed with a cutout portion, the multi-beam source unit being designed so as to be set to a scanning optical system on the assumption that the plural light emitting points are arranged in the direction of a predetermined standard design line when they are present on a virtual straight line defined by the said cutout portion, the method comprising measuring an arranged state of the light emitting points with resect to the standard design line on the basis of beam spots on an image surface corresponding to an image recording surface and rotating the multi-beam laser diode about an optical a of the scanning optical system to align the arrangement direction of the light emitting points with the direction of the standard design line.
In the second aspect of the present invention there is provided, in combination with the above first aspect, a multi-beam source unit adjusting method wherein the virtual straight line is defined by a concave or convex portion as an engaging portion for positioning formed in the stem.
According to the above first and send aspects of the invention, since an arranged state of the light emitting points with respect to the standard design line is measured on the ba of beam spots on an image surface and the multi-beam laser diode is rotated for adjustment about the optical axis of the scanning optical system, the arrangement direction of the light emitting points can be aligned with the direction of the standard design line easily.
In the third aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam soured unit adjusting method wherein on the image surface corresponding to the image recording surface an arrangement direction of the light emitting points with respect to the standard design line is determined on the basis of a straight line obtained by joining two beam spots corresponding to two light emitting points located remotest from each other out of the light emitting points.
According to the multi-beam source unit adjusting method in the above third aspect of the invention, since an arrangement direction of the light emitting points is determined using only the light emitting points located remotest from each other, it is possible to determine a light emitting points arrangement direction of the multi-beam laser diode easily.
In the fourth aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjusting method wherein on the image surface corresponding to the image recording surface there are measured relative positions of beam spots corresponding to the light emitting points to determine an approximate straight line on which the light emitting points can be regarded as being present, and an arrangement direction of the light emitting points is determined by the approximate straight line.
In the fifth aspect of the present invention there is provided, in combination with the above fourth aspect, a multi-beam source unit adjusting method wherein the approximate straight line is obtained by a method of least squares.
In the sixth aspect of the present invention there is provided, in combination with the above first or second aspect, a multi-beam source unit adjust method wherein, on the image surface corresponding to the image recording surface, relative positions in the horizontal scanning direction of beam spots corresponding to the light emitting points are measured in terms of relative angle positions with respect to the standard design line to determine a maximum deviation in the horizontal scanning direction among the beam spots, then the multi-beam laser diode is rotated to measure relative positions of the beam spots in the horizontal direction at different relative angle positions, thereby determining a maximum deviation in the horizontal scanning direction among the beam spots, these operations are repeated to measure an arranged state of the light emitting points, and an arrangement direction of the light emitting points is determined by a relative angle position corresponding to the smallest maximum deviation.
According to the multi-beam source unit adjusting methods in the above fourth to sixth aspects of the invention, the arrangement direction of the multi-beam source unit can be aligned with the direction of the standard design line with a high accuracy.
In the seventh aspect of the present invention there is provided, in combustion with the above first or second aspect, a multi-beam source unit adjusting method wherein the arrangement direction of the light emitting points is substantially parallel to the vertical scanning direction.
According to the multi-beam source unit adjusting method in the above seventh aspect of the invention, since the arrangement direction of the light emitting points can be aligned with the vertical scanning direction before mounting the same unit to a body portion of an image forming apparatus, it is possible to dispense with such an operation as adjusting the beam spot pitch in the vertical scanning direction after mounting the multi-beam source unit to the body portion of the image forming apparatus and it is not necessary to adopt a complicated construction such that for correcting write start positions displaced in the horizontal scanning direction due to the beam spot pitch adjustment, sensors for detecting laser beam scan positions are provided for laser beams respectively on the image forming apparatus body side and the write start position is controlled for each laser beam, nor is it necessary to adopt a complicated construction such that a time lag of each laser beam in the horizontal scanning direction is measured and a control is made for driving each laser beam with use of a delay circuit. Thus, it is possible to reduce the number of components used, shorten the assembling time required for mounting to the body portion of the image forming apparatus, and attain the reduction of cost. It is also possible to attain the simplification of control using software.
In the eighth aspect of the present invention there is provided, in combination with the first or second aspect, a multi-beam source unit adjusting method wherein the multi-beam source unit is provided with a base member, the base member supporting the multi-beam laser diode rotatably and having a fitting cylinder which defines a rotational center, the multi-beam source unit is also provided with a mounting bracket to be mounted to a body portion of an image forming apparatus, the mounting bracket having a horizontal scanning direction reference plane to be confronted with a horizontal scanning direction reference plane formed in the body portion of the forming apparatus and also having a fitting hole to be fitted on the fitting cylinder, the multi-beam source unit is further provided with an engaging piece for engagement with an engaging portion for positioning and a pressing spring piece for pressing the stem, the engaging piece being brought into engagement with the engaging portion for positioning, and the base member being rotated while being supported by the mounting bracket to adjust the arrangement direction of the light emitting points substantially in parallel with the vertical scanning direction.
According to the multi-beam source unit adjusting method in the eighth aspect of the invention, since the multi-beam source unit can be adjusted alone before being mounted to the body portion of the image forming apparatus, it is possible to simplify the mounting work for mounting the multi-beam source unit to the image forming apparatus body.
In the ninth aspect of the present invention there is provided a multi-beam source unit adjusting device comprising a base member adapted to be positioned and fixed, the base member being provided with a multi-beam laser diode having a plurality of light emitting points and capable of emitting multi-laser beams and also provided with a collimator lens for collimating the multi-laser beams; an image pickup device on which the multi-laser beams are projected for measuring an arrangement direction of the light emitting points on the basis of beam spots and for rotating the multi-beam laser diode about an optical axis of an optical system; and a focusing lens disposed between the image pickup device and the collimator lens to condense and focus the multi-laser beams onto an image pickup surface of the image pickup device, wherein a front-side focal position of the focusing lens is substantially coincident with a rear-side focal position of the collimator lens.
According to the multi-beam source unit adjusting device in the ninth aspect of the invention, since the multi-laser beams from all the light emitting points can be enlarged, condensed and focused substantially onto the image pickup surface, the position of each beam spot can be detected with a high accuracy.
In the tenth aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein the image device is a CCD camera and the using lens is disposed so that a focused area of a beam spot of each multi-laser beam on an image pickup surface of the CCD camera is ten times or more as large as a pixel area.
According to the multi-beam source unit adjusting device in the tenth aspect of the invention, the resolving performance on the image pickup surface can be improved and a central position of each beam spot can be calculated with a high accuracy.
In the eleventh aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein for controlling in such a manner as to give substantially equal respective outputs of the multi-laser beams there is provided a control circuit which controls luminous outputs of N number of light emitting points so that the sum total of outputs of the light emitting points is N times as large as a detected outputs of one of the laser beam from any one of the N number of light emitting points.
According to the multi-beam source unit adjusting device in the eleventh aspect, since it is possible to equalize the luminous outputs of the light emitting points, the beam spots on the pickup surface can be made equal in size and it is possible to calculate a central position of each beam pot with a high accuracy.
In the twelfth aspect of the present invention there is provided, in combination with the above ninth aspect, a multi-beam source unit adjusting device wherein a central position of each of the beam spots is a centroid position of a CCD pixel output corresponding to the beam spot.
According to the multi-beam source unit adjusting device in the twelfth aspect of the invention, the center of each beam spot can be measured with a high accuracy.
In the thirteenth aspect of the present invention there is provided, in combination with the above twelfth aspect, a multi-beam source unit adjusting device wherein the centroid position of each of the beam spots is determined by subtracting 1/e of a maximum value of the CCD pixel output and performing calculation using a CCD pixel output larger than zero.
According to the multi-beam source unit adjusting device in the thirteenth aspect, the center of each beam spot can be determined with a high accuracy even if the beam spot is out of shape.
In the fourteenth aspect of the present invention there is provided a method for assembling a multi-beam source unit, the multi-beam source unit including a multi-beam laser diode capable of emitting multi-laser beams from a plurality of light emitting points, the multi-beam laser diode having a stem formed with a cutout portion, the multi-beam source unit also including a collimator lens for collimating the multi-laser beams, the multi-beam source unit being designed so as to be set to a scanning optic system on the assumption that the light emitting points are arranged in the direction of a predetermined standard design line when they are present on a virtual straight line defined by the said cutout portion, wherein an arranged state of the light emitting points with respect to the standard design line is measured on the basis of beam spots formed on an image surface corresponding to an image recording surface and the multi-beam laser diode is rotated about an optical axis of the scanning optical system to align the direction of arrangement of the light emitting points with the direction of the standard design line, the method comprising a step of position the multi-beam laser diode to a base member, the base member supporting the multi-beam laser diode rotatably and having a fitting cylinder which defines a rotational center, and fixing the multi-beam laser diode with use of a pressing spring piece, a positioning step of positioning the collimator lens with respect to the multi-beam laser diode, a calculating step of measuring spot positions of the laser beams emitted from the multi-beam laser diode and calculating central positions thereof and a rotation adjusting step of adjusting the rotation of the base member with the multi-beam laser diode attached thereto in such a manner that the direction of arrangement of the beam spots is aligned with the standard design line direction.